The Problem: Defined as tan or brownish discoloration on the paint surface. Tannins are present in many woods (cedar, redwood and mahogany), and can migrate or bleed through to the surface leaving yellowish-brown spots. This problem is more prevalent when lighter paint colors are used.
Causes: Failing to prime and seal raw wood surfaces before painting. Using a primer that is not resistant to stains. And the presence of high levels of humidity and other forms of moisture that can cause tannins to stain through the paint surface.
How-to-fix: Locate and remedy any sources of excess moisture (see efflorescence). Remove loose paint and clean the surface with a commercial grade solution. Thoroughly rinse and dry the surface prior to applying a high-quality stain resistant oil-based or latex exterior primer. In extreme cases, a second coat of primer may be required. Finally, repaint using a high-quality latex paint.